This invention relates to a method and apparatus for measuring electrical energy and, in particular, a method and apparatus for measuring electrical energy consumed at a rate of consumption.
An electrical utility provides consumers with an electrical energy service and measures each consumer's use of the service. Providing this service, the utility incurs production, distribution and other operating costs. These costs are recovered from consumers, preferably in the form of an equitable pro rata contribution from each consumer based upon those costs attributable to each consumer. It is recognized that a portion of the costs attributable to any one consumer depends upon two quantities: the total amount of electrical energy used by the consumer and the rate at which the consumer uses that energy. Hence, it is desirable for utilities to physically measure both quantities.
There are meters which can directly or indirectly measure these quantities. A rotating disc watthour meter is typically used to directly measure total energy in units of watthours. The construction and operation of conventional rotating disc meters is well known and can be found in Electrical Metermen's Handbook, Chapter 7, Edison Electric Institute, 7th ed., 1965.
Rate of energy usage, commonly termed power or demand, is equal to the instantaneous product of voltage and current. Because demand in an instantaneous quantity which can widely fluctuate during a given time interval, it is desirable to have a method of indirectly measuring this quantity. One such method is to measure electrical energy consumed during a period when demand exceeds a predetermined level. The conventional rotating disc meter can be equipped with a differential gearing mechanism to perform this measurement. Such differential gear excess consumption meters are disclosed in the reference, supra, beginning at page 312. However, the required differential gear mechanism is complex and expensive.
There are also known means for locking a rotor disc. The reference, supra, at p. 107 describes relatively small, non-adjustable anti-creep holes. These holes are placed in the rotor disc to distort the eddy currents in the disc and thereby create a relatively small locking torque. In order to lock the rotor under load condition, the size of the anti-creep holes has to be increased. However, increasing the size of the holes further distorts the rotor disc eddy currents. This upsets otherwise reliable meter characteristics thereby reducing meter accuracy.
The foregoing disadvantages are overcome by an excess consumption watthour meter which contains an amount of mildly magnetic material coupled to the rotor disc.